Step 2: Set maximum acceptable [P], [P]f, following the introduction of
fish culture. Assuming no other developments or criteria take
precedence, then 60 mg m-3 is chosen as target [P]f.

Step 3: Determine Δ[P]

Δ[P]

= [P]f - [P]i = 45 mg m-3

Since[P]

=

Lfish

=

Rfish is taken to approximate R calculated from the equation of
Larsen and Mercier (1976) (see Table 23)

Step 4: Since the lake has a surface area of 106 m2, the total acceptable
loading = 0.833 x 106 g y-1

∴ the tonnage of fish that can be produced, assuming a P loading of
17.7 kg tonne-1 (see Table 16)

This value should be used as a pre-development guide to the
carrying capacity of the lake. However a monitoring programme
must be implemented, and actual production levels adjusted in the
light of information collected on water quality - principally
algal biomass and O2 levels.

Vi, L and A can be determined by direct measurement, whilst F can be
derived from published data on buccal cavity size, and gill opercular
beating rates (see Hoar and Randall, 1976). The following calculations
are based on typical values: -

This is very much higher than the typical stocking values of 5 – 50 fish
m-3 for extensive cage culture. However, the model assumes that the fish
themselves do not contribute to the drag forces exerted on currents
flowing through cages, or that conversely the movement of fishes in the
cages may increase circulation. The relative importance of these two
factors remains unknown. Also, it is assumed that the fish fully
evacuates its buccal cavity on each occasion, which is unlikely.

Model B O2 requirements

If the current velocity through the cages is computed, and the O2 concentration
of the water known, then the supply of O2 to the fish cage can
be calculated. If the O2 requirements of the caged fish are computed,
assuming worst possible conditions (high temperatures, small fish,
requirements following a meal), then we can calculate the appropriate
stocking density: -